Amplifier circuit having a high degree of common mode rejection

ABSTRACT

The disclosed unity gain amplifier circuit includes current supplies, current sinks, input, output and feedback transistors and an inverting gain stage. The negative feedback transistor stabilizes the quiescent signals and enables the magnitude of the amplifier output signal to follow the magnitude of the amplifier input signal. The gain stage, which is connected between the current supplies, the current sinks, and the input and feedback transistors, utilizes undesired signals occurring therein to cancel each other so that the undesired signals do not adversely effect the magnitude of the amplifier output signal.

BACKGROUND OF THE INVENTION

There are many applications in modern day electronic equipment foramplifier circuits having a high degree of common mode rejection whichdiminishes the creation of unwanted signal components at the outputterminals thereof. More specifically, such amplifiers are often desiredfor implementation in integrated circuit form for utilization inautomotive applications. One such application relating to engine fuelcontrol requires a stable, unity gain amplifier having a high inputimpedance and a high degree of common mode rejection. Such amplifiersmust provide an output voltage having precisely the same magnitude as ananalog input voltage occurring across a storage capacitor.

One prior art monolithic amplifier circuit configuration includes an NPNdifferential amplifier having two differential NPN transistors with acurrent source connected to the common emitters thereof. A furthercurrent source is connected to the collector of the output differentialtransistor and a further NPN transistor is connected between the furthercurrent source and the control electrode of the output differentialtransistor. This amplifier configuration meets the requirements of theprevious paragraph except for the requirement relating to common moderejection. More specifically, the internal impedances associated withthe current sources and with the input differential transistor tend tocause undesired imbalance in the signals of the circuit. As themagnitude of the input voltage applied to the base of the inputdifferential transistor increases, the voltage at the common emitterpoint of the differential transistor also increases. This produces achange in the magnitude of the current of the differential amplifiercurrent source. Also, the driving voltage produces different changes inthe magnitudes of the currents flowing through the input differentialtransistor and in the output current source. Since these changes incurrent magnitudes are not equal, the emitter current of thedifferential pair tends to undesirably change as a function of thechange in the magnitude of the input voltage. As a result, thedifferential or common mode voltage across the differential amplifierchanges as a function of the input voltage magnitude change therebycausing the magnitude of the output voltage of the amplifier to notprecisely follow the magnitude of the analog input voltage as required.

Another prior art circuit utilizes a differential amplifier includinginside and outside pairs of PNP transistors. A current source isconnected to the emitters of the inside pair of PNP transistors. Adifferential-to-single ended converter including a diode and a NPNtransistor is connected between the collectors of the inside pair of PNPtransistors. An output NPN transistor is connected between the converterand the output differential amplifier transistor. Thedifferential-to-single ended converter tends to force the emittercurrents of the inside pair of differential transistors to be equalindependent of the value of the current of the current source. Theinside pair of differential transistors maintains balance in the outsidepair of PNP transistors. Thus, the PNP differential amplifier provides ahigh degree of common mode rejection.

However, the foregoing prior art PNP differential amplifier tends tohave relatively high input currents because of the low betas of the PNPtransistors, which can undesirably load the driving circuit. Thisproblem is particularly troublesome if the circuit is fabricated inmonolithic integrated circuit form. Such loading can be disadvantageous,for example, if the driving circuit includes a storage capacitor havingan analog voltage of a precise magnitude stored thereon since theloading will change the magnitude of the analog voltage. Furthermore,this PNP amplifier tends to limit the excursion of the output voltagethereof because of the base-to-emitter junction drops of the transistorsconnected between its output terminal and the positive power supplyconductor. However, the most serious disadvantage is that this amplifierhas two stages of gain which tend to oscillate when the amplifier isconnected in a unity gain configuration. Therefore, large compensationcapacitors are required to prevent oscillation. Such capacitors on themonolithic integrated circuit are costly or if provided externally willrequire additional pin-outs which increase the costs associated with themanufacture and use of the integrated circuit.

SUMMARY OF THE INVENTION

One object of the invention is to provide a simple amplifier circuithaving a high degree of common mode rejection capability.

Another object is to provide a stable amplifier circuit suitable formanufacture in integrated circuit form which has a high degree of commonmode rejection capability, high input impedance and which is stableenough for connection in a unity gain configuration without requiring alarge on chip compensation capacitance.

In brief, the amplifier circuit of one embodiment provides an outputsignal at an output terminal thereof having a magnitude which preciselyfollows the magnitude of an input signal at an input terminal thereof.The amplifier circuit configuration includes first and second currentsupplies and a signal cancellation circuit. The first current supply iscoupled to the output terminal of the amplifier and causes a firstundesired signal which tends to adversely effect the magnitude of theamplifier output signal. The second current supply is also coupled tothe amplifier output terminal and causes a second undesired signal whichtends to adversely effect the magnitude of the amplifier output signal.The signal cancellation circuit has one terminal coupled to the firstcurrent supply and another terminal coupled to the second currentsupply. The signal cancellation circuit utilizes the first and secondundesired signals to cancel each other so that they do not adverselyeffect the magnitude of the amplifier output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE is a schematic diagram of an amplifier circuit of oneembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Amplifier circuit 10 of FIG. 1, which can be provided in either discreteor monolithic form, has an input terminal 12, a positive supplyconductor 14 and a negative supply or ground conductor 16. NPN inputtransistor 18 includes a base electrode connected to input terminal 12,a collector electrode connected to positive supply conductor 14 and anemitter electrode connected to circuit node 20. NPN current sinktransistors 22 and 24 have base electrodes connected together and to theanode electrode of bias diode 25. Resistor 27 is connected between theanode of diode 25 and positive power supply conductor 14. The emitterelectrodes of transistors 22 and 24 and the cathode of diode 25 are allconnected to negative supply conductor 16. Transistors 22 and 24 aredesigned to have matched electrical characteristics. Diode 25 providesequal bias to transistors 22 and 24 in response to the current throughresistor 27.

PNP current source or supply transistors 26 and 28 have emitterelectrodes which are connected to supply conductor 14, base electrodeswhich are connected together and to the cathode of bias diode 30.Resistor 32 is connected between the cathode of diode 30 and thenegative supply conductor 16. The emitter electrodes of transistors 26and 28 and the anode of diode 30 are all connected to positive powersupply conductor 14. Transistors 26 and 28 are designed to have matchedelectrical characteristics. Diode 30 provides equal bias to currentsource transistors 26 and 28.

Gain stage NPN transistor 34 includes a collector electrode connected tothe collector electrode of current supply transistor 26 and to the baseelectrode of output transistor 36. Transistor 34 also includes anemitter electrode connected to the collector electrode of current sinktransistor 22 and a base electrode connected to both the collectorelectrode of current supply transistor 28 and to the anode of diode 38.NPN output transistor 36 includes a collector electrode which isconnected to power supply conductor 14 and an emitter electrode which isconnected to output terminal 40 of amplifier 10, to one terminal ofresistor 42 and to the base electrode of feedback transistor 44. Theemitter electrode of NPN transistor 44 is connected to the cathodeelectrode of diode 38 and to the collector electrode of current sinktransistor 24. The collector electrode of transistor 44 is alsoconnected to positive power supply conductor 14. Transistors 18 and 44are designed to have the same electrical characteristics as each other.

QUIESCENT OPERATION

Current source or supply transistors 26 and 28, which have similargeometries and structures, are each biased by diode 30 and resistor 32to provide constant currents having a predetermined magnitude of I. Thevalue of resistor 27 and the geometries and structures of current sinktransistors 22,, 24 and diode 25 are arranged so that current sinktransistor 22, which is connected to the emitters of transistors 18 and34, and current sink transistor 24, which is connected to the cathode ofdiode 38 and the emitter electrode of transistor 44, each draw a currenthaving a magnitude of 2I. Thus, input transistor 18 and feedbacktransistor 44 each also conduct a quiescent current of I.

The base-to-emitter junctions of NPN transistors 18, 34 and 44 and thejunction of diode 38 are all designed to have similar areas andelectrical characteristics. Since currents of the same magnitude (I)flows through all of these junctions, the base-to-emitter junctionvoltages (φ) of these devices are all substantially equal.

Consequently, amplifier output terminal 40 tends to have the samequiescent potential as input terminal 12. In particular, thebase-to-emitter junction of transistor 18 drops the input bias voltageby its base-to-emitter junction voltage (φ), transistor 34 then raisesthe voltage at node 20 by its base-to-emitter junction voltage (φ),diode 38 then drops the voltage at the base of transistor 34 by itsanode-to-cathode junction voltage (φ), and transistor 44 raises thevoltage at node 52 by the amount of its base-to-emitter junction voltage(φ).

Transistor 44 provides negative feedback which tends to stabilize thequiescent voltage. If the voltage at terminal 40 undesirably decreases,then the voltage across resistor 42 decreases. Consequently transistor44 becomes less conductive. Thus, diode 38 shunts more of the currentfrom the base of transistor 34 to satisfy sink transistor 24. Transistor34 then becomes less conductive which increases the collector voltagethereof. Transistor 36 responds by providing more current to resistor 42and to the load.

The input currents of amplifier 10 can be arranged to have lowmagnitudes by causing the currents in amplifier 10 to have lowmagnitudes. In addition, the use of NPN input transistors withconventional IC fabrication techniques also helps to reduce the inputcurrent. Thus, circuit 10 can be arranged to provide minimal currentdrain from the power supply connected across conductors 14 and 16 andminimal input bias current loading.

INPUT IMPEDANCE

Since NPN transistor 18 has a relatively high beta as compared to amonolithic PNP transistor, amplifier 10 provides a higher inputimpedance than monolithic amplifiers having PNP input transistors for agiven amplifier bias current. Thus circuit 10 does not undesirably loada driver circuit connected to input terminal 12. This high inputimpedance is particularly important if a storage capacitor, forinstance, having a precision analog control voltage developedthereacross is connected between input terminal 12 and ground. Suchcapacitors are used in automotive circuitry associated with engine fuelcontrol, for instance.

DYNAMIC OPERATION

As a dynamic signal voltage of increasing magnitude is applied toamplifier input terminal 12, transistor 18 is rendered more conductivethereby supplying a current having a greater magnitude than I.Consequently, transistor 34 supplies a current having a magnitude ofless than I because transistor 22 will not sink current having amagnitude greater than 2I. The collector voltage of transistor 34increases in response to transistor 34 becoming less conductive. Thustransistor 36 is rendered more conductive thereby providing more currentto the load which is connected to amplifier output terminal 40.Consequently, the magnitude of the voltage cross the load alsoincreases.

If the magnitude of the load voltage tends to increase more than themagnitude of the voltage at terminal 12, then transistor 44 is renderedmore conductive thereby supplying a current having greater magnitude.Consequently, diode 38 conducts a current having a lower magnitude sincetransistor 24 will only sink a current of 2I. As a result, current fromcurrent supply 28 is diverted into the base of transistor 34 whichrenders transistor 34 more conductive thereby reducing the drive totransistor 36 which becomes less conductive and equalizes the magnitudesof the input and load voltages. Therefore, transistor 44 providesnegative feedback which causes the magnitude of the voltage at outputterminal 40 to be equal to and to precisely follow the magnitude of thevoltage at terminal 12. Thus, transistor 44 enables amplifier 10 to haveunity voltage gain.

Transistors 18, 36 and 44 are all connected in emitter-followerconfigurations which provide no voltage gain but these transistors doprovide current gain. Transistor 34 provides the open loop invertedvoltage gain stage. Consequently, circuit 10 is quite stable with asingle gain stage and does not require large internal capacitors or anyexternal components for frequency compensation.

COMMON MODE REJECTION

Dotted resistances 53 and 54, respectively, represent the internalimpedances of the current supplies including transistors 26 and 28,respectively. Dotted resistances 56 and 58, respectively, represent theinternal resistances of the current sinks including transistors 22 and24, respectively. Dotted resistances 64 and 65, respectively, representthe internal resistances of the input and feedback NPN transistors.Since transistors 26 and 28 are matched, the resistance of resistances53 and 54 are approximately equal to each other; and since transistors22 and 24 are matched, the resistance of resistances 56 and 58 are alsoapproximately equal to each other. Also since transistors 18 and 44 arematched, the resistances 64 and 65 are approximately equal to eachother.

Current supply transistor 26 has a terminal 59 which is coupled throughtransistor 36 to output terminal 40 of amplifier circuit. As has beenpreviously explained, during dynamic operation, the voltage at thecollector electrode of transistor 34 tends to vary in magnitude. Morespecifically, the voltage at the collector electrode of transistor 34differs from the output voltage by only the base-to-emitter voltageacross transistor 36. Since the voltage at positive supply conductor 14has a fixed magnitude and since the voltage at terminal 59 varies, thecurrent through resistance 53 tends to vary and adversely affect themagnitude of the output signal at output terminal 40 of the amplifiercircuit. Similarly, current supply transistor 28 has an output terminal61 which is coupled through transistors 34 and 36 to output terminal 40of amplifier circuit 10. During dynamic operation, the voltage atterminal 61 also changes which causes an undesired signal throughresistance 54 which tends to adversely effect the magnitude of theoutput signal at output terminal 40 of amplifier circuit 10.

Transistor 34 and diode 38 are included in a signal cancellation circuit63 which is coupled to terminals 59 of the current supply circuitincluding transistor 26 and to terminal 61 of the current supply circuitincluding transistor 28. The undesired signal at terminal 59 tends toincrease the drive to output transistor 36 by a predetermined amountwhile the undesired signal at terminal 61 tends to render transistor 34more conductive which reduces the drive to transistor 36 by the samepredetermined amount. Thus, signal cancellation circuit 63 utilizes theundesired signals from transistors 26 and 28, which are of a similarnature to each other, to cancel each other so that such undesiredsignals do not adversely affect the magnitude of the output signal atoutput terminal 40 of amplifier circuit 10.

Transistor 18 is coupled through transistors 34 and 36 to outputterminal 40 of amplifier circuit 10. During dynamic operation,transistor 18 creates undesired signals at the emitter electrode oftransistor 34 and across resistance 56 of current sink transistor 22which tend to adversely effect the magnitude of the output signal atoutput terminal 40 of amplifier circuit 10. Likewise, as the magnitudeof the output signal on output terminal 40 changes, feedback transistor44 tends to cause undesired signals across resistance 58 of current sinktransistor 24 and at the cathode of diode 38.

Signal cancellation circuit 63 utilizes these undesired signals oftransistors 22 and 24, which are similar to each other, to cancel eachother so that these signals do not adversely effect the magnitude of theoutput signal at output terminal 40 of amplifier circuit 10. Forinstance, as the input signal increases in magnitude, the emittervoltages of transistors 18 and 44 both tend to increase by the sameamount because the magnitude of the output signal at terminal 40 tendsto follow the magnitude of the input signal at terminal 12. Theincreasing voltage at the emitter of transistor 34 tends to rendertransistor 34 less conductive by a certain amount while the increase involtage at terminal 52 tends to cause transistor 34 to be moreconductive by the same amount. Cconsequently, the conduction oftransistor 34 is not changed. The undesirably increasing currents ofcurrent sink transistors 22 and 24 are also cancelled by signalcancellation circuit 63.

Resistor 64 and resistor 65 which are associated with input transistor18 and feedback transistor 44 respectively each produces currents whichtend to create undesired signals which adversely affects the magnitudeof the output signal at output terminal 40. However signal cancellingcircuit 63 also utilizes these undesired signals of transistors 18 and44 to cancel each other so that these signals do not adversely affectthe magnitude of the output signal at output terminal 40. For instance,as the input signal increases in magnitude, the emitter voltage oftransistor 18 and 44 both tend to follow the input change as previouslydescribed. The increasing voltage at terminal 20 causes a decrease incurrent through resistor 64 which increases the current in transistor 34which decreases the drive current to the base of transistor 36. However,the increase in voltage at terminal 52, causes a decrease in currentthrough resistor 65 which increases the current in diode 38 whichdecreases the drive to transistor 34 which increases the drive to thebase of transistor 36. Thus the changing currents associated withtransistors 18 and 44 are also cancelled by signal cancellation circuit63.

What has been described is embodiment 10, a simple, unity gain amplifiersuitable for fabrication in monolithic integrated circuit form. Thecircuit provides high common mode rejection capability and the stableconfiguration of circuit 10 has a high input impedance.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

I claim:
 1. An amplifier circuit for providing an output signal at anoutput terminal thereof having a magnitude which precisely follows themagnitude of an input signal at an input terminal thereof, including incombination:first current supply means having predetermined electricalcharacteristics and at least one terminal, said first current supplymeans being coupled to the output terminal of the amplifier circuit,said first current supply means causing a first undesired signal whichtends to affect the output signal at the output terminal of theamplifier circuit; second current supply means having electricalcharacteristics matched to said predetermined electrical characteristicsof said first current supply means and at least one terminal, saidsecond current supply means being coupled to the output terminal of theamplifier circuit, said second current supply means causing a secondundesired signal having substantially the same magnitude as the firstundesired signal which tends to affect the magnitude of the outputsignal at the output terminal of the amplifier circuit; and signalcancellation circuit means having a current mirror means which respondsto a current at a first terminal thereof to provide another current ofsubstantially the same magnitude at a second terminal thereof, saidfirst terminal of said current mirror means being coupled to saidterminal of said first current supply means and said second terminal ofsaid current mirror means being coupled to said terminal of said secondcurrent supply means, said current mirror means utilizing said first andsecond undesired signals of the same magnitude to cancel each other sothat said first and second undesired signals do not adversely affect theoutput signal at the output terminal of the amplifier circuit.
 2. Theamplifier circuit of claim 1 further including output electron controlmeans coupled between said signal cancellation circuit means and theoutput terminal of the amplifier circuit.
 3. The amplifier circuit ofclaim 1 further including feedback electron control means connectedbetween the output terminal of the amplifier circuit and said signalcancellation circuit means.
 4. The amplifier circuit of claim 1 furtherincluding in combination:first electron control means having at least acontrol electrode and an output electrode, said first electron controlmeans being coupled to the output terminal of the amplifier circuit,said first electron control means causing a third undesired signal whichtends to effect the output signal at the output terminal of theamplifier circuit; second electron control means having at least acontrol electrode and an output electrode, said second electron controlmeans being coupled to the output terminal of the amplifier circuit,said second electron control means causing a fourth undesired signalwhich tends to effect the output signal at the output terminal of theamplifier circuit; and said current mirror means of said signalcancellation circuit further having a third terminal and a fourthterminal, said third terminal being coupled to said output electrode ofsaid first electron control means, said fourth terminal being coupled tosaid output electrode of said second electron control means, saidcurrent mirror means utilizing said third and fourth undesired signalsto cancel each other so that said third and fourth undesired signals donot adversely effect the output signal at the output terminal of theamplifier circuit.
 5. The amplifier circuit of claim 4 further includingin combination:first current sink means having at least one terminal,said first current sink means being coupled to said output terminal ofthe amplifier circuit, said first current sink means causing a fifthundesired signal which tends to effect the output signal at the outputterminal of the amplifier circuit; second current sink means having atleast one terminal, said second current sink means being coupled to theoutput terminal of the amplifier circuit, said second current sink meanscausing a sixth undesired signal which tends to effect the output signalat the output terminal of the amplifier circuit; and said third terminalof said current mirror means of said signal cancellation circuit beingcoupled to said terminal of said first current sink means and saidfourth terminal of said current mirror means being coupled to saidterminal of said second current sink means, said cancellation circuitmeans utilizing said fifth and sixth undesired signals to cancel eachother so that said fifth and sixth undesired signals do not adverselyaffect the output signal at the output terminal of the amplifiercircuit.
 6. A unity gain amplifier circuit having a high degree ofcommon mode rejection for providing an output signal at an outputterminal thereof having a magnitude which substantially equals andprecisely follows the magnitude of an input signal at an input terminalthereof, including in combination:first current supply means having atleast one terminal, said first current supply means being coupled to theoutput terminal of the amplifier circuit, said first current supplymeans causing a first undesired signal which tends to adversely effectthe magnitude of the output signal at the output terminal of theamplifier circuit; second current supply means having at least oneterminal, said second current supply means being coupled to the outputterminal of the amplifier circuit, said second current supply meanscausing a second undesired signal which tends to adversely effect themagnitude of the output signal at the output terminal of the amplifiercircuit; first electron control means having at least a controlelectrode and an output electrode, said first electron control meansbeing coupled to the output terminal of the amplifier circuit, saidfirst electron control means causing a third undesired signal whichtends to adversely effect the magnitude of the output signal at theoutput terminal of the amplifier circuit; second electron control meanshaving at least a control electrode and an output electrode, said secondelectron control means being coupled to the output terminal of theamplifier circuit, said second electron control means causing a fourthundesired signal which tends to adversely effect the magnitude of theoutput signal at the output terminal of the amplifier circuit; andsignal cancellation circuit means having a current mirror means with afirst terminal coupled to said terminal of said first current supplymeans, a second terminal coupled to said terminal of said second currentsupply means, a third terminal coupled to said output electrode of saidfirst electron control means, and a fourth terminal coupled to saidoutput electrode of said second electron control means, said signalcancellation circuit means utilizing said first and second undesiredsignals to cancel each other and said third and fourth undesired signalsto cancel each other so that said undesired signals do not adverselyeffect the magnitude of the output signal at the output terminal of theamplifier circuit.
 7. The amplifier circuit of claim 6 wherein:saidfirst electron control means includes a bipolar transistor having baseand emitter electrodes respectively corresponding to said control andoutput electrodes; and said second electron control means includes abipolar transistor having base and emitter electrodes, respectively,corresponding to said control and output electrodes thereof.
 8. Theamplifier circuit of claim 6 wherein said current mirror means includesa bipolar transistor and a diode connected to said bipolar transistor.9. The amplifier circuit of claim 6 further including incombination:first current sink means having at least one terminal, saidfirst current sink means being coupled to said output terminal of theamplifier circuit, said first current sink means causing a fifthundesired signal which tends to adversely effect the magnitude of theoutput signal at the output terminal of the amplifier circuit; secondcurrent sink means having at least one terminal, said second currentsink means being coupled to the output terminal of the amplifiercircuit, said second current sink means causing a sixth undesired signalwhich tends to adversely effect the magnitude of the output signal atthe output terminal of the amplifier circuit; and said third terminal ofsaid current mirror means of said signal cancellation circuit beingcoupled to said terminal of said first current sink means and saidfourth terminal of said current mirror means being coupled to saidterminal of said second current sink means, said cancellation circuitmeans utilizing said fifth and sixth undesired signals to cancel eachother so that said fifth and sixth undesired signals do not effect themagnitude of the output signal at the output terminal of the amplifiercircuit.
 10. The amplifier circuit of claim 6 further including anoutput transistor coupled between said first terminal of said currentmirror means and the output terminal of the amplifier circuit.